1. Field of the Invention
The present invention refers to a measuring device capable of conducting the following measurements;
Measurement of the flow rate of a fluid;
Recognition of the kind of a fluid;
Measurement of the temperature of a fluid;
Measurement and detection of impurities contained in a fluid;
Evaluation of the thermal environment; and
Evaluation of the thermal effects.
2. Description of the Prior Art
A thermistor-using device is known as one for measuring the flow. In this apparatus, heat absorption by a fluid leading to a fall in the temperature of a thermistor section is used. Generally, because the heat quantity escaped from the thermistor section, when the thermistor section is in contact with a fluid, depends on the flow rate or flow velocity, the output of the thermistor bears a certain correlation to the flow rate, by the use of which correlation the flow rate can be computed from the output of the thermistor.
The flow rate is given by the product of the sectional area and flow velocity of a fluid. For example, assuming a fluid flows in a circular pipe having the inside diameter r at a flow velocity V, the flow rate becomes V.pi.r.sup.2. Hereinafter, description will be focused on the flow rate, but the flow rate and flow velocity can be simultaneously evaluated if the sectional area of the fluid is known.
Generally, the thermistor means a semiconductor having a large negative temperature coefficient. However, the thermistor originally means a thermally sensitive resistor and is not limited in particular by the positivity or negativity of a temperature coefficient or materials. Thus, metals, such as Pt, having a positive temperature coefficient may be called thermistors.
The elements using materials of temperature dependent resistance, like thermistors, are known generally as temperature measuring resistors or temperature sensitive elements, further as thermosensible elements or resistance thermometers. On the contrary, materials with temperature dependent resistance may be called those having a thermistor function. Hereinafter, materials with temperature dependent resistor will be referred to as temperature-measuring resistors.
Also, there is a system using a loss dependent on the flow rate in the quantify of heat from the relevant resistive heating unit on exposing a resistive heating unit heated by Joule's heat to a fluid. According to this system, the flow rate can be computed by measuring a current flowing through the resistive heating unit.
Further, there is another system in which allowing the quantity of heat to be absorbed by a fluid from a heating unit in contact with the fluid, the quantity of heat conveyed by the fluid is measured with a temperature-measuring resistor (e.g. Pt thermistor) provided separately.
In these systems, a greater quantity of heat absorbed by a fluid is effective for obtaining a higher sensitivity. And, to increasing a response speed, minimizing the heat capacity of a temperature-measuring resistor part is required.
The flow rate measuring device as indicated above have a drawback in that the measurable range of a flow rate is narrow, i.e., the dynamic range is narrow. Specifically, there is a problem in that an accurate measurement can be made only in the range between 20 sccm and 300 sccm or 200 sccm and 2000 sccm.
The inventors of the present invention considered that the above drawback is caused by the following problems:
(1) Since the temperature-measuring resistor is subjected to a thermally extremely unstable conditions, a response to heat is poor in linearity and cannot follow a wide range of thermal change.
(2) In conjunction with the above (1), especially a method for heating is difficult and no effective heating can be done in a wide range of flow rate.
(3) If the heat capacity of a temperature-measuring resistor is made smaller for increasing a response speed, a large quantity of heat cannot be dealt with.
The above (1) originates in that it is difficult to realize a structure capable of allowing the quantity of heat to be effectively absorbed by a fluid from a temperature-measuring resistor and simultaneously supplying the quantity of heat to the temperature-measuring resistor in a wide range of flow rate.
There is another problem that changes in temperature of an environment of the device or a fluid to be measured affects the operation of the device, for example, hindering the accuracy of the measurement.
Furthermore, if a battery drive considered, it becomes necessary to minimize the consumed power. For example, in the case of applying a flow rate measuring device to a household gas meter, it is required to enable a battery to operate as a power supply for more than several years. In such cases, a flow rate measuring device operable at a low power consumption is to be demanded. However, for a lower power consumption, there arises a new problem in that the measuring sensitivity and the measuring accuracy gives way and no required characteristic can be obtained.